In positioning a head of a disk device to a target track by a seek operation, control inputs to a voice coil motor (VCM) for driving the head are provided in velocity control mode, settling mode, and track following mode. The settling mode, together with the track following mode is often called position control mode. Because each of these modes controls differently, an overshoot would occur to result in a longer time for positioning the head to the target track unless initialization is adequate when the velocity control mode is switched to the settling mode or when the settling mode is switched to the track following mode. In the conventional positioning systems, there is generally nothing special to be done in mode switching from velocity control to settling, and a value which is stored when the head rests on the target track is used as an initial output value of an integrator when the settling mode is switched to the track following mode. However, such a method does not guarantee fast positioning.
Japanese Published Unexamined Patent Application (PUPA) No. 2-244467 discloses a control method for positioning in a magnetic disk device which performs seek control for moving a head to a target track and following control for positioning a head at the center of the target track, wherein a part of calculation of a digital filter used in the following control is processed in parallel with the seek control during a predetermined period immediately before the mode switching time. According to the method, the most suitable initial value of a state variable is provided when the seek control is switched to the following control, so that an overshoot of a head is prevented. However, this control method needs parallel calculations and is complex in comparison with the present invention which uses only state variables at the mode switching time.
Japanese PUPA No. 2-304782 discloses a control method in a magnetic disk device which uses velocity control for moving a head to a target track and position control for positioning the head to the target track, wherein an initial manipulated variable and cut-off frequency in the position control are adjusted depending on an initial position and initial velocity at the switching time from the velocity control to the position control so that an overshoot is prevented. However, it is undesirable in terms of accuracy to make use of velocity. Besides, even the cut-off frequency must be adjusted in this method, which becomes complicated in comparison with the present invention.
As described above, the conventional methods for preventing an overshoot of a head need extra calculations and operations. Therefore, it is desired to initialize more simply at the mode switching time.
An object of the present invention is to enable fast positioning of the head by determining an initial value dynamically from state variables of a system at the mode switching time.
A further object of the present invention is to enable fast positioning of the head by adding a feedforward value which is determined by state variables at the time when the velocity control mode is switched to the settling mode.
A still further object of the present invention is to enable fast positioning of the head by using, as an initial value of an integrator, a value which is determined by state variables at the time when the settling mode is switched to the track following mode.